Nanocrystalline cellulose suspensions produced by sulfuric acid hydrolysis (i.e. the H-NCC form) are not dispersible in water or other aqueous solvents once they have been fully dried. When the proton counterion H+ is exchanged for monovalent cationic counterions M+, dried forms of M-NCC can spontaneously redisperse in water to give suspensions with properties similar to those of the native suspensions after sonication treatment [1].
As examples of water-dispersible M-NCC, suspensions of Na-NCC, K-NCC, Cs-NCC, NH4-NCC, Et4N-NCC, (tetraethylammonium+-NCC) Bu3MeN-NCC (tributylmethylammonium+-NCC), Bu4N-NCC (tetrabutylammonium+-NCC), and Hex4N-NCC (tetrahexylammonium+-NCC) are produced by titrating H-NCC suspensions with the solutions of the appropriate hydroxide to neutral pH. The resulting M-NCC suspensions may be dried by several methods, including freeze-drying, spray drying and casting into self supporting films or coatings on substrates. These dried forms of NCC disperse in deionized water to give colloidal suspensions of NCC.
While H-NCC films have the advantage of being non-dispersible in water not shared by the aforementioned M-NCC films, they do not display other advantages associated with M-NCC films.
Industrially, aqueous suspensions of NCC will have to be dried in order to transport and store large quantities of NCC. The dried NCC must therefore be fully redispersible in water for applications that require the use of aqueous suspensions having the full expression of the unique properties of NCC. The drying process must also not interfere with the intrinsic/inherent properties of the NCC particles themselves and/or those of the resulting aqueous NCC suspension. The sodium form of NCC, Na-NCC (obtained by stoichiometrically exchanging the protons of the original acid form H-NCC with sodium, e.g., by neutralization with NaOH), is the form of choice for this purpose. It is completely redispersible in water, much more thermally stable than H-NCC (thermal decomposition begins at 300° C. for Na-NCC as opposed to 180° C. for H-NCC) and does not undergo self-catalyzed desulfation (loss of negatively charged surface sulfate ester groups which result in the electrostatic stability of aqueous NCC suspensions) or degradation of the cellulose upon drying and during prolonged storage of the dried material.
It appears likely that Na-NCC will be widely used in industry as the initial form of NCC from which products will be manufactured. However, its dispersibility in water may not be desirable in many applications, while the thermal and chemical stability may well be. As such, dried Na-NCC has some advantages but also some disadvantages relative to dried H-NCC.